The paper describes the ecology and nature conservation interest of the aquatic ecosystems of urban Britain. The paucity of published information is attributed to the historical lack of interest by the government's ecological and nature conservation agencies, research workers, and those concerned with the management of urban green space. Urban water bodies are classified according to their origins and use, as modified by age, size, water quality and level of public use. The plants and animals to be found in each category are described. Most of the information relates to London and to birds. However, London is atypical in many respects because of its size, large estuary, large parks, an active natural history society, and because it is the capital city. Apart from Milton Keynes (a new town) and some areas of the West Midlands, there is very little information about the ecosystems of other British towns and cities. The data indicate the importance of urban water bodies for the conservation of wildlife in Britain. The potential for improving the value for nature by ecological engineering is demonstrated. The diffused management responsibilities are described together with the need to impose a discipline on the maintenance authorities by the preparation of management plans in order to ensure continuity and consistency of treatment. A programme of research is advocated on the basis of the re-allocation of existing funds. The prime objective of such a programme is the acquisition of pragmatic information that will allow planning, design and maintenance operations to respond to inevitable development whilst maintaining a high-quality environment.